This invention relates to the elimination or suppression of deep trap centers in III-V semiconductors and in particular the suppression of deep donor states or DX centers in III-V semiconductors, such as GaAs, and AlGaAs alloys, and GaAsP GaAsSb, AlGaAsP, and GaInAsP alloys.
Group III-V alloys, such as Al.sub.x Ga.sub.1-x As, are increasingly becoming important materials because of their applications in optoelectronics and in heterojunction devices, such as, high electron mobility transistors (HEMT). In the case of n-Al.sub.x Ga.sub.1-x As, particularly where x.gtoreq.22%, there is a sharp decrease in conductivity, and a persistent photoconductivity effect resulting from the formation of deep traps, which adversely affect device operations. Group IV or Group VI n-type dopants simultaneously provide a shallow donor near the conduction band and a deep donor level, which acts as an electron trap state and has been associated with a defect center labeled as DX. The shallow levels become largely unoccupied and inactive as a result of deep trap formation in Al.sub.x Ga.sub.1-x As alloys where x.gtoreq.22%. See, for example, the articles of D. V. Lang et al. entitled "Trapping Characteristics and a Donor-Complex (DX) Model for the Persistent-Photoconductivity Trapping Center in Te-Doped Al.sub.x Ga.sub.1-x As", Physical Review B, Vol. 19, pp. 1015-1030, 1979; "Large-Lattice-Relaxation Model for Persistent PhotoConductivity in Compound Semiconductors", Physical Review Letters, Vol. 39, pp. 635-639, 1977; and D. V. Lang at pp. 489-539 in the book "Deep Centers in Semiconductors", edited by S. T. Pantelides (Gordon and Breach, New York, 1986). The DX center arises when Al.sub.x Ga.sub.1-x As alloys are doped with either group IV or group VI atoms which preferentially substitute for Ga (or Al) and As atoms, respectively, i.e., substitutional doping occurs, which to those skilled in the art is the common and preferred manner of doping in most III-V semiconductors, vis a vis interstitial doping, which is not regarded as being as stable, suitable or, defect free. Present thinking in the art does not consider interstitials as suitable shallow-donor dopants because of a belief that these donors are unstable and are apt to diffuse rapidly away from the bulk and onto the surface where they are not wanted, and it leans toward their elimination with the conviction that substitutional atoms in combination with other forms of treatment represent the best mode for decreasing, if not eliminating, defect problems in the lattice. See, as an example, U.S. Pat. No. 4,745,448 issued May 17, 1988.
Recent work has revealed that the DX center is also present as a metastable center in n-type GaAs. Its density, for example, in GaAs can be increased in various ways, e.g., through: (1) application of pressures in excess of 20 kbars (See, for example, M. Mizuta et al., "Direct Evidence for the DX Center Being a Substitutional Donor in AlGaAs Alloy System", Japanese Journal of Applied Physics, Vol. 24, p. L143 et seq., 1985; N. Lifshitz et al., "Pressure and Compositional Dependence of the Hall Coefficient in Al.sub.x Ga.sub.1-x As and Their Significance" Physical Review B, Vol. 21, pp. 670-678, 1980; (2) an increase in the dopant concentration to about 10.sup.19 /cm.sup.3 (See, for example, T. N. Theis et al., "Electron Localization by a Metastable Donor Level in n-GaAs: A New Mechanism Limiting the Free-Carrier Density", Physical Review Letters, Vol. 60, pp. 361-364, 1988); or (3) simply by raising the temperature (See, for example, J. R. Kirtley et al., "Noise Spectroscopy of Deep Level (DX) Centers in GaAs-Al.sub.x Ga.sub.1-x As Heterostructures", Journal of Applied Physics, Vol. 63, pp. 1541-1548, 1980.
The DX center exhibits a very large Stokes shift of approximately 1 eV between its thermal and optical ionization energies. Lang, supra, and others, such as Mooney (P. M. Mooney et al., "Evidence for Large Lattice Relaxation at the DX Center in Si-Doped Al.sub.x Ga.sub.1-x As", Physical Review B, Vol. 37, pp. 8298-8307, 1988), attribute this to a large lattice relaxation. Lang employed a simple configuration coordinate diagram to show that the large optical gap was consistent with the measured emission and capture barriers for this center. Since, at the time in 1977, a large relaxation appeared unlikely to occur for an isolated donor, it was also proposed that the DX center resulted from the formation of a complex consisting of the donor (D) and an unknown or unidentified defect (X), possibly an As vacancy. This donor-vacancy model has been examined by Van Vechten and collaborators, for example in: R. J. Higgins et al., "Mobility Enhancement of Modulation-Doped Materials by Low-Temperature Optical Annealing of Spacer-Layer Defect Charge State" Physical Review B, Vol. 36, pp. 2707-2712, 1987 ; Van Vechten et al. for the case of the analogous "M center" in InP: J. A. Van Vechten and J. F. Wager "Consequences of Anion Vacancy Nearest-Neighbor Hopping in III-V Compound Semiconductors: Drift in InP Metal-Insulator-Semiconductor Field Effect Transistors", Journal of Applied Physics, Vol. 57, pp. 1956-1960, 1985; J. F. Wager and J. A. Van Vechten, "Atomic Model for the M Center in InP", Physical Review B, Vol. 32, pp. 5251-5258, 1985.
The defect-complex model for DX, while attractive in explaining many properties of the DX center, has been recently challenged and the possibility that the DX center arises per se from the substitutional donor itself has received increasing attention. The prevailing view now ascribes DX formation to a Jahn-Teller distortion of the donor with either large or small lattice relaxations [See T. N. Morgan, "Theory of the DX Center in Al.sub.x Ga.sub.1-x As and GaAs Crystals", Physical Review B, Vol. 34, pp. 2664-2669, 1986; H. P. Hjalmarson et al. "Deep Donor Model for the Persistent Photoconductivity Effect" Applied Physics Letters, Vol. 48, pp. 656-658, 1986.]. Results from most recent experiments (Mooney et al., supra) tend to favor a large lattice relaxation model.
Recent work of Peter Yu and coworkers [M. F. Li, P. Y. Yu, W. Shan, W. Hansen, and E. R. Weber, "Effect of Boron on the Deep Donors (DX Centers in GaAs:Si" (to be published in Applied Physics Letters)] have demonstrated that when B is introduced into GaAs doped with Si, the DX centers disappear and new deep donors with significantly reduced binding energies and capture barrier heights appear. They explain this change to be caused by a direct interaction of B with Si which results from some type of B-Si complex formation arising from the lattice strain induced by the smaller B atom when it becomes substitutionally incorporated in the lattice. See M. F. Li et al., "Effect of Boron on the Pressure Induced Deep Donors in GaAs:Si", Meeting and Bulletin of the American Physical Society, Vol. 33(3), p. 439, Mar. 21-25, 1988, New Orleans, La. Also, P. Basmaji et al. "Enhancement of the Free Carrier Density in Ga.sub.1-x Al.sub.x As Grown by Metalorganic Vapor Phase Epitaxy Under High Temperature Growth Conditions" Physica status solidi (a), Vol. 100, pp. K41-K45, 1987, found that the free electron concentration in n-type Al.sub.x GA.sub.1-x As increased dramatically under high As/(Ga+Al) ratios of 95 and growth temperatures of 950.degree. C. in metalorganic vapor phase epitaxy. This change was attributed to the increase in growth temperature that possibly brought about a greater solubility of shallow donors, or possibly a decrease in the number of deep donor levels, or possibly by an unknown change in the nature of the deep donor levels.
What is needed is a fundamental understanding, on a microscopic basis, of what causes the formation of the DX center and then means which can be employed to prevent the formation of the DX center in the lattice structure of III-V semiconductors.